A plasminogen activator from pig heart has been purified to a specific activity in excess of 100,000 CTA units/milligrams protein, which is comparable to the specific activity of the most purified human urokinase (UK) preparations. Molecular weight determinations by gel filtration indicate molecular weights of 51,500 and 48,000 daltons for the two active species of pig heart tissue activator (PHTA). Preparations of PHTA catalyze the hydrolysis of acetyl-L-lysine methyl ester and acetyl-glycyl-L-lysine methyl ester, but the Km's for PHTA are about one order of magnitude higher than for UK. Inhibition studies using proteolytic enzyme inhibitors demonstrate other differences between PHTA and UK. Rabbit antisera to PHTA has been used to demonstrate immunological identity between PHTA and the circulating plasminogen activator of the pig. Further work will be directed to determining if the minor components seen in highly purified PHTA preparations are different activators or whether they are degradative products of the major component. Other pig tissues (adrenal, kidney, lung) will be fractionated to obtain tissue activators, and their physical, chemical and biological properties will be studied to determine if activators from different tissues are similar. Plasminogen activation by various tissue activators, UK and streptokinase will be studied to determine if the mode of activation is similar, and whether proteolytic enzyme inhibitors like trasylol inhibit the activation of plasminogen. Further studies on the purification and properties of human tissue activators will be conducted, employing the methods described for pig heart tissue activator.